1. Field of the Invention
The invention relates to an electronic exposure control system for a solid state image sensor, in particular for active pixel MOS image sensor (CMOS), and a method for using the same. More particularly, the invention relates to a xe2x80x9crolling readoutxe2x80x9d for the CMOS image sensor using row decoders wherein the pixel exposure time is different from the frame time and rows can be read out non-sequentially, one row at a time.
2. Background of the Invention
There are basically two types of solid state image sensors used for image acquisition, charge coupled devices (CCD) and CMOS sensors. Although CCD image sensors have found wide-spread applications, for example in electronic movie and still cameras, the process used for their fabrication is specialized and different from the process used for manufacturing other commonly used integrated electronic circuits (IC), such as microprocessors, employing MOS technology.
The individual radiation sensing elements (pixels) in a CMOS array are basically reverse-biased semiconductor junctions of the type normally used to form source and drain regions of MOS transistors. Radiation is detected by first applying a large reverse bias to the device (reset), by subsequently isolating the device from the reverse bias voltage, and by then measuring the decrease in the charge stored in the reverse-bias junction capacitance of the device by either sensing the reverse-bias potential (voltage-mediated sensing or read-out) or by integrating the reverse-bias current flowing through the device (current-mediated sensing or read-out). The reset function can also be executed by either a voltage source (voltage reset) or a current source (current reset). The features of a current-mediated active-pixel image sensing device are described in detail in U.S. application Ser. No. 08/595,981 by McGrath et al., entitled xe2x80x9cCurrent-mediated active-pixel image sensing device with current resetxe2x80x9d which is assigned to the same assignee as the present invention and which is incorporated herein by reference.
Electronic image sensors, not unlike conventional film, have a certain exposure range; if the exposure time is too short, almost no charge is drained from the reverse-biased junction, resulting in a very small voltage drop which is difficult to measure and consequently results in a large noise-to-signal ratio; if the exposure time is too long, almost all charge will be drained for the majority of pixels, thereby providing little image content or image contrast. It is therefore important to provide shutter means for limiting the exposure of the pixels to incoming radiation. This can be accomplished either by mechanical means, for example by providing the imaging system incorporating the electronic image sensor with a mechanical shutter, like the shutter in a photographic camera using film, as described, for example, in U.S. Pat. No. 5,341,220, or by electronic means, such as variable pre-amplifiers or by controlling the time between reset and read-out of pixels as a function of the intensity of the incoming radiation. The latter approach is commonly referred to as an xe2x80x9celectronic shutter.xe2x80x9d
An electronic shutter for a linear CCD array is, for example, described in U.S. Pat. No. 5,303,052 wherein a shutter timing unit generates between read-out pulses a shutter pulse for draining to a special drain region excess charge accumulated in the CCD pixels. A similar concept for CCD video area image sensors is described in U.S. Pat. No. 5,247,367 wherein the shutter speed is controlled by adaptively varying the duration of light storage of the CCD for every vertical frame period in response to the radiative intensity.
The pixel signals of CMOS image sensors are conventionally read out differently from CCD image sensors. All pixels in each row are commonly addressed by digital circuitry, such as a shift register, one row at a time, either to be reset or to be selected for read out. When a row is enabled for read out, columns are preferably sequentially selected and the pixel signal of pixel at a commonly selected row and column is read out by a charge sense amplifiers connected to the column lines. Such a read-out method is described, for example, in U.S. Pat. No. 5,345,266.
In many cases, shift registers are used in the digital circuitry for row (and also column addressing). Shift register addressing allows simple operation of the sensor. Addressing a row requires placing a xe2x80x9conexe2x80x9d at the input of the shift register and then shifting the appropriate number of rows.
It is possible to provide with row addressing via a shift register, a pixel exposure time which is shorter than the frame time. This is accomplished by resetting a first row at the beginning of a frame using a first shift register, by then consecutively resetting succeeding rows, one row at a time, with the first shift register, and by reading the first row at a later time which is less than the frame time, using a second shift register which is delayed with respect to the first shift register by a certain number of rows which is less than the number of rows in the frame. A shutter of this type is described, for example, in U.S. Pat. No. 5,410,348, using two row shift registers. Shift registers, however, are disadvantageous in that once the read-out time has been set by specifying the delay time between the two shift registers, this delay time cannot be changed at a later time unless the shift registers are reset to their start address, making it difficult to read out partial frames and to dynamically adapt the xe2x80x9celectronic exposure timexe2x80x9d to changing illumination conditions.
The disadvantages of the method described above can be obviated by employing decoders instead of shift registers. With decoder addressing, a specific row is selected by placing a word at the input to the decoder. One word each is required per horizontal or vertical direction. Decoder addressing requires more I/O but is more versatile. Regions of interest can be read out as well as various other non-sequential read-out schemes for exposure control can be envisioned.
With the foregoing in mind, it is therefore an object of the invention to provide an improved control system for controlling the exposure of pixels of a CMOS image sensor to incident optical radiation. Preferably, the control system uses decoders for row-wise and column-wise addressing the pixels. The CMOS image sensors can be a current-mediated active-pixel image sensing device.
It is another object of the invention to provide decoder addressing for the rows of radiation-sensitive pixels of a CMOS image sensor, wherein the pixel exposure time can be less or greater than the frame time.
It is a further object of the invention to provide decoder addressing for the rows of radiation-sensitive pixels of a CMOS image sensor, wherein the pixel exposure time can be changed easily during one frame time.
It is another object of the invention to enable read-out of areas of the image sensor which are smaller than the total imaging area of the image sensor.
It is another object of the invention to enable read-out of non-sequential rows of radiation-sensitive pixels of a CMOS image sensor, e.g. of interlaced rows, during one frame time.
These objectives are achieved by connecting the input of a CMOS image sensor having two input lines per row, a first row input line for resetting and a second row input line for selecting pixels in one row to be read out by a column decoder, in parallel to respective output terminals of a row decoder. The input of the row decoder is connected to a reset and a select counter, respectively, via respective buffer amplifiers. The reset counter and the select counter, respectively, are connected to an xe2x80x9cintelligentxe2x80x9d controller to receive input signals from the output of the controller. The input signals include row clock pulses, start and stop addresses of the rows defining the row-wise imaging area, respective reset enable pulses and select enable pulses, a load enable pulse for loading the start and stop addresses into the respective counters and a preset incremental value for the row count. The select enable pulse is delayed with respect to the reset enable pulse by a certain number of row clock cycles defining the xe2x80x9cheightxe2x80x9d of the rolling focal plane shutter and thus the respective pixel integration time.
The input signals received by the reset counter and the select counter, respectively, include the row addresses and the reset and select enabling signals; these addresses and enabling signals are alternately switched during one row clock cycle by the respective buffer amplifier. Pixels in columns can be addressed and the signals stored therein can be read out by a column decoder, which can also be controlled by the xe2x80x9cintelligentxe2x80x9d controller. The exposure information contained in the pixels can also be used to modify the output signals supplied by the xe2x80x9cintelligentxe2x80x9d controller. Both the exposure time of pixels and the area of the image sensor addressed for reset and readout can thus be changed in response to the image content.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.